#include<iostream>
#include<algorithm>
using namespace std;
typedef long long LL;
const int N = 5e4 + 10;
int l, n, m;
int a[N];

bool check(int x)
{
	int cnt = 0;
	int idx = 0;
	for (int i = 1; i <= n + 1; i++) {
		if (a[i] - idx < x) cnt++;
		else idx = a[i];
	}
	return cnt <= m;
}
int main()
{
	cin >> l >> n >> m;
	int ma = 0;
	for (int i = 1; i <= n; i++) {
		cin >> a[i];
		ma = max(ma, a[i] - a[i - 1]);
	}
	a[n + 1] = l;
	ma = max(ma, a[n + 1] - a[n]);
	int l = 0, r = 1e9 + 1;
	while (l + 1 < r) {
		int mid = l + r >> 1;
		if (check(mid))l = mid;
		else r = mid;
	}
	cout << l << endl;
	return 0;
}